text: The Black Sea is a commonly used modern model for understanding biogeochemical processes that occur under euxinic conditions. It is thought to represent the conditions of Earth's proto-oceans and thus assists in the interpretation of oceanic proxies. Black Sea sediment contains redox reactions to depths of tens of meters, compared to single centimeters in the open ocean. This unique feature is important for understanding the behavior of the redox cascade under euxinic conditions.
  The only connection between the open ocean and the Black Sea is the Bosphorus Strait, through which dense Mediterranean waters are imported. Subsequently, numerous rivers, such as the Danube, Don, Dnieper, and Dniester, drain fresh water into the Black Sea, which floats on top of the more dense Mediterranean water, causing a strong, stratified water column. This stratification is maintained by a strong pycnocline which restricts ventilation of deep waters and results in an intermediate layer called the chemocline, a sharp boundary separating oxic surface waters from anoxic bottom waters usually between 50m and 100m depth, with interannual variation attributed to large scale changes in temperature. Well-mixed, oxic conditions exist above the chemocline and sulfidic conditions are dominant below. Surface oxygen and deep water sulfide do not overlap via vertical mixing, but horizontal entrainment of oxygenated waters and vertical mixing of oxidized manganese into sulfidic waters may occur near the Bosphorus Strait inlet. Manganese and iron oxides likely oxidize hydrogen sulfide near the chemocline, resulting in the decrease in HS concentrations as one approaches the chemocline from below.